The present invention relates to testing the continuity of electrical conductors without any physical contact, and more particularly to a system and method of using an electron beam to test conductive networks in an insulating specimen, the networks having terminations on at least one surface of the specimen. In one application, the system is employed to test networks in modules formed of multi-layer ceramic (MLC) laminates.
It is desirable in the manufacture of integrated circuit devices to test for defects at an early stage of fabrication in order to minimize the cost of repairing such defects and to maximize the yield of operable devices. The small size and high density of present devices creates a need for a test system other than conventional mechanical probe systems which rely on physical contact with the conductor terminations. In certain applications, such physical contact can damage the device under test. With the introduction of multi-layer packaging such as MLC, mechanical and optical techniques are also limited by resolution.
MLC packages are particularly exemplary of testing difficulties often encountered. Such packages are formed of multiple layers of unfired ceramic substrates having conductive material deposited thereon. Contact from one layer to another is achieved by vias punched in each layer and filled with conductive material.
The unfired substrates have a paste-like consistency, so that physical contact would damage the specimen. The individual substrates are subsequently pressed together to form laminates and then fired to produce the MLC module. The fired modules are more susceptible to mechanical probing, although reliable probing is difficult to achieve because of surface irregularities, the small size of the features being probed, and displacement of the features due to shrinkage of the module during firing.
The use of electron beam systems to provide noncontact testing has been proposed. For example, U.S. Pat. No. 3,764,898 proposed a method for bombarding at least one end of a conductor with an electron beam, measuring the secondary emission of electrons at the one end to detect the potential thereat, and measuring the current that flows through the conductor to determine its state of continuity. U.S. Pat. Nos. 4,415,851 and 4,417,203 disclose systems for testing networks including both top to bottom and top to top connections. The systems include two electron beam flood guns and a scanning beam gun. The scanning beam is arranged to scan the top surface of the specimen under test. One flood gun irradiates the bottom surface for top to bottom testing, and the other flood gun irradiates the top surface for testing top to top connections.
Chang et al., in "Tri-Potential Method for Testing Electrical Opens and Shorts in Multi-Layer Ceramic Packaging Modules," IBM Technical Disclosure Bulletin vol. 24, no. 11A, pp. 5388-5390, April 1982, disclose a method for MLC testing using localized charging wherein a scanning electron beam addresses test points on the top surface of the module at two different potentials, one for charging and one for detection. A second beam, which can be either a scanning beam or a flood beam, is used to charge the back side of the module at a third potential. Other systems for such testing are described by Pfeiffer et al. in "Contactless Electrical Testing of Large Area Specimens Using Electron Beams," J. Vac. Sci. Techno., vol. 19, no. 4, pp. 1014-1018. Golladay et al. suggested the addition of a negatively biased stabilizer grid between the back side flood gun and the specimen to increase the uniformity of charging and to control the potential to which the networks are charged, in "Stabilizer Grid for Contrast Enhancement in Contactless Testing of MLC Modules," IBM Technical Disclosure Bulletin, vol. 25, no. 12, pp. 6621-6623, May 1983.
The system of Chang provides a means of testing electrical conductors without physical contact; however, it is limited in that all conductive materials cannot be tested. In Chang, the conductors must be charged by an electron beam with an energy higher than that corresponding to the second crossover potential of the material under test. This condition is readily fulfilled for materials such as molybdenum, nickel and copper, which require a charge potential of about 4 KV and a read or detection potential of about 2 KV. However, for materials such as lead-tin and gold, with crossover potentials of about 5-6 KV and 8-9 KV, respectively, significantly higher charge beam potentials are required. Even if such potentials could be achieved in a production test system, it would be difficult to meet the required high speed operation since switching of the high beam potential must preferably be done within approximately 100 microseconds.
In U.S. Pat. No. 4,417,203 a flood gun is used to create a negative charge on all conductors on the top surface of the specimen being tested. Since a fixed flood potential is used, the beam potential limitations of Chang et al. are not a problem. After the specimen is negatively charged by the flood beam, a selected network is scanned by a focused probe beam which discharges all conductors in the selected network. If there is an open circuit the corresponding terminal will still be charged. The other conductive networks in the specimen are then tested. If the first point of a subsequently scanned network is already discharged, this is an indication of a short circuit between the conductor connected to that terminal and one of the previously tested networks. One disadvantage of this technique is that the discharged networks tend to become negatively charged again due to the attraction to the networks of any low energy electrons present in the system. This is because the discharged networks are positive relative to the rest of the specimen being tested. This instability or negative charging can mask the detection of short circuit defects.